This invention relates to a metal threaded fastener having insulating plastic thereon and to a method and apparatus for making the same.
The present invention will be described in connection with a preferred embodiment in the form of electrical connector fastener which has a head substantially encapsulated in a plastic and which has metal washers with peripheral edges thereof covered by plastic and disposed on the upper portion of the shank of the fastener. The lower portion of the fastener is threaded, usually with a self-threading, spiral screw thread. The outer edges of the washers are coated with plastic while inner metal contact surfaces on adjacent washers contact wire clips or wraps of wire to establish an electrical connection between the conductors and the inner metal contact surfaces of the washers. Usually, the head of the fastener is provided with a slotted head to be driven by a screwdriver.
These self-threading, electrical terminal screws have not heretofore been manufactured with polygonal driving heads because this plastic head section should have substantially the same strength as a standard metal fastener without the plastic with respect to being driven by a power tool or manual wrench. This is difficult because as the plastic tends to tear away or separate from the metal head at the high torques applied to the screw head with a hexagonal-headed tool or the plastic corners shear or become "rolled over" or rounded off so that the tool cannot turn the head.
A number of hybrid plastics have been molded to metal screw head ends without great commercial success because the plastic tends to separate from the metal or the corners of plastic head tend to round off. Another problem is that if one tries to make the plastic for the head too hard or too brittle to increase corners of the plastic heads in resisting deformation or shearing into the rounded off configuration at high torque loads, then the plastic has a tendency to be brittle and to crack or to chip at the thin outer layer on the edges of the metallic washers.
Thus, it is an object of the present invention to provide a new and improved threaded fastener of the foregoing kind with an improved plastic head resistant to tearing away from the metal fastener and to rounding off at the corners at normally applied torques.
Another object of the invention to provide a plastic head portion that is stronger and less ductile than the encapsulated edges of the washers on the fastener so that the more ductile edges of the washers not chip or crack away from the metal edges on the washers.
A still further object of the invention is to provide a method and apparatus for the manufacture of this improved fastener.